This invention relates to resistor inks that are normally applied as a paste coating to an insulative substrate and fired to form a resistor film. More particularly this invention relates to resistor ink additives that are intended to effect a desired modification in the temperature coefficient of resistance (TCR) of the fired resistor film.
Such resistor inks may consist of metal particles mixed with glass frit in a liquid vehicle. Others contain metal resinates, with or without glass frit, which resinates decompose at firing leaving a deposit of metal or metal oxide either in a glass matrix or as an all metal (including metal oxides) film. A survey of resinate film technology including resistor films is provided in Electrical Applications of Thin Films Produced by Metallo Organic Deposition by C. Y. Kuo, International Microelectronics Symposium 1973 -- Oct. 22-24.
The electrical conductive path through such resistor films is through the metal and/or metal oxides. The temperature coefficient of resistance of these films tends to be positive when the path is predominantly metal and negative when predominantly metal oxide. It is well known to choose the ratios of particular metals and metal oxides in the film to achieve a desired TCR. Adjustments in the metal and metal oxide content of the film for purposes of adjusting the TCR are normally accompanied by corresponding changes in sheet resistivity, and current noise level. It thus becomes necessary when so adjusting the TCR to make other compensating adjustments, for example in film thickness or glass frit content. This involves an arduous and difficult procedure requiring extensive experimentation and evaluation in the formulation and processing of any new ink system for a particular application.
Many high resistivity ink systems that are suitable for making small high ohmic value (e.g. greater than 50,000 ohms per square) resistors, exhibit a negative TCR. The sheet resistivity of such resistors is particularly sensitive to additives that may be appropriate for an upward (positive) adjustment of their TCR. Most known additives have the effect of a downward (negative) adjustment of TCR as well as significantly increasing the resistivity.
It is therefore an object of this invention to provide an additive in a resistance material that adjusts the TCR in a positive direction.
It is a further object of this invention to provide an additive that adjusts the TCR in a positive direction and simultaneously either has a minor effect on the resistivity or causes the resistivity to decrease by a small amount.
It is a further object of the present invention to provide a resistor having a resistivity of greater than 50,000 ohms per square and a near zero temperature coefficient of resistance.